Damage free laser patterning of transparent layers for forming doped regions on a solar cell substrate

ABSTRACT

Laser patterning methods utilize a laser absorbent hard mask in combination with wet etching to form patterned solar cell doped regions to improve cell efficiency by avoiding laser ablation of an underlying semiconductor substrate associated with ablation of an overlying transparent passivation layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication 61/816,830 filed on Apr. 29, 2013 which is herebyincorporated by reference in its entirety.

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/90,115 filed May 29, 2013 and Ser. No. 14/137,172 filed Dec.20, 2013, which are all hereby incorporated by reference in theirentirety.

FIELD

The present disclosure relates in general to the fields of solarphotovoltaic (PV) cells, and more particularly to laser processing ofphotovoltaic solar cell substrates.

BACKGROUND

Laser processing offers several advantages in terms of efficiencyenhancement and manufacturing cost reduction for high-performance,high-efficiency solar cell processing. Firstly, advanced crystallinesilicon solar cells may benefit from having the dimensions of thecritical features such as electrical contacts be much smaller than thecurrent industrial practice. For front contacted solar cells the contactarea of the front metallization to the emitter as well as the contactarea of the back metal to the base needs to be low (or the contact arearatios should be fairly small, preferably much below 10%). For an allback-contact, back junction solar cell, where the emitter and baseregions forming the p/n junction and the metallization are on the sameside (the cell backside opposite the sunny side), the dimensions of thevarious features are typically small for high efficiency. In these cellswhere typically the emitter and base regions form alternate stripes, thewidth of these regions (in particular the width of the base contact)tends to be small. Also, the dimensions of the metal contacts to theseregions tend to be proportionally small. The metallization connecting tothe emitter and base regions then needs to be patterned to acorrespondingly finer scale. Generally, lithography and laser processingare the technologies that have the relatively fine resolution capabilityto provide the small dimensions and the control required. Of thesetechniques, only laser processing offers the low cost advantage requiredin solar cell making. While lithography requires consumables such asphotoresist and subsequent resist developer and stripper (which add tothe process cost and complexity), laser processing is a non-contact,dry, direct write method and does not require any material consumables,making it a simpler and lower cost process for solar cell fabrication.Moreover, laser processing is an excellent choice for environmentallybenign manufacturing since it is an all-dry process which does not useany consumables such as chemicals.

Further, to reduce the cost of solar cells there is a push to reduce thethickness of the crystalline silicon used and also at the same timeincrease the cell area for more power per cell and lower manufacturingcost per watt. Laser processing is suitable for these thin wafers andthin-film cell substrates as it is a completely non-contact, dry processand can be easily scaled to larger cell sizes.

Laser processing is also attractive as it is generally a “green” andenvironmentally benign process, not requiring or using poisonouschemicals or gases. With suitable selection of the laser and theprocessing system, laser processing presents the possibility of veryhigh productivity with a very low cost of ownership.

Despite these advantages, the use of laser processing in crystallinesilicon solar cell making has been limited because laser processes thatprovide high performance cells have not been developed. Disclosed hereare laser processes using schemes that are tailored for each keyapplication to produce solar cells with high efficiency. Specificembodiments are also disclosed for applications of laser processing inmanufacturing thin-film crystalline silicon solar cells, such as thosemanufactured using sub-50-micron silicon substrates formed by epitaxialsilicon growth.

SUMMARY

Various laser processing schemes are disclosed herein for producinghetero junction and homo-junction solar cells. The methods include baseand emitter contact opening, front and back surface field formation,selective doping, metal ablation, annealing, and passivation. Inparticular, laser patterning methods utilizing a laser absorbent hardmask in combination with wet etching to form patterned solar cell dopedregions are provided which may further improve cell efficiency bycompletely avoiding laser ablation of an underlying semiconductorsubstrate associated with ablation of an overlying transparentpassivation layer.

Also, laser processing schemes are disclosed that are suitable forselective amorphous silicon ablation and selective doping for heterojunction solar cells. These laser processing techniques may be appliedto semiconductor substrates, including crystalline silicon substrates,and further including crystalline silicon substrates which aremanufactured either through wire saw wafering methods or via epitaxialdeposition processes, that are either planar ortextured/three-dimensional. These techniques are highly suited to thincrystalline semiconductor, including thin crystalline silicon films.

Laser processing schemes are disclosed that meet the requirements ofbase to emitter isolation (including but not limited to shallow trenchisolation) for all back-contact homo-junction emitter solar cells (suchas high-efficiency back-contact crystalline silicon solar cells),opening for base doping, and base and emitter contact opening (withcontrolled small contact area ratios, for instance substantially below10% contact area ratio, for reduced contact recombination losses andincreased cell efficiency), selective doping (such as for base and/oremitter contact doping), and metal ablation (formation of patternedmetallization layers such as creating the patterned metallization seedlayer on a thin-film monocrystalline silicon solar cell prior tosubsequent attachment of a backplane to the cell and its release from areusable host template) for both front-contact and allback-contact/back-junction homo-junction emitter solar cells. Also,laser processing schemes are disclosed that are suitable for selectiveamorphous silicon ablation and oxide (such as a transparent conductiveoxide (TCO)) ablation, and metal ablation for metal patterning forhetero junction solar cells (such as back-contact solar cells comprisinghetero junction amorphous silicon emitter on monocrystalline siliconbase). These laser processing techniques may be applied to semiconductorsubstrates, including crystalline silicon substrates, and furtherincluding crystalline silicon substrates which are manufactured eitherthrough wire saw wafering methods or using epitaxial depositionprocesses, which may be either planar or textured/three-dimensional,where the three-dimensional substrates may be obtained using epitaxialsilicon lift-off techniques using porous silicon seed/release layers orother types of sacrificial release layers. These techniques are highlysuited to thin crystalline semiconductor, including thin crystallinesilicon films obtained using epitaxial silicon deposition on a templatecomprising a porous silicon release layer or other techniques known inthe industry.

An all back-contact homo-junction solar cell may be formed in thecrystalline silicon substrate, wherein laser processing is used toperform one or a combination of the following: micromachine or patternthe emitter and base regions including base to emitter isolation as wellas openings for base, provide selective doping of emitter and base, makeopenings to base and emitter for metal contacts, provide metalpatterning, provide annealing, and provide passivation. A frontcontacted homo-junction (emitter) solar cell may be made using laserprocessing for selective doping of emitter and making openings for metalcontacts for both frontside and backside metallization. A heterojunction all back-contact back-contact solar cell may be made usinglaser processing for defining the base region and conductive oxideisolation.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the disclosed subject matterwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings, in which like referencenumerals indicate like features and wherein:

FIG. 1 shows a scanning electron microscope (SEM) image of a shallowtrench made in silicon for application in an all back contact backjunction solar cell, in accordance with the present disclosure;

FIG. 2 shows a profile of a shallow trench in silicon for application inall back contact back junction solar cells;

FIGS. 3A-3D show the procedure for selecting the laser fluence to obtainreduced damage silicon dioxide (or oxide) ablation. FIG. 3A shows thedependence of the size of the ablation spot on the laser fluence; FIG.3B shows irregular delamination of oxide; FIG. 3C shows a damage-freespot; and FIG. 3D shows highly damaged silicon in the spot opening;

FIG. 4 shows substantially parallel rows of contacts opened in oxideusing pulsed laser ablation in accordance with the present disclosure;

FIG. 5 shows a screenshot with oxide ablation spots for metal contacts;

FIGS. 6A and 6B show the laser-ablated area formed by making ablationspots that are overlapped in both the x and y-direction; FIG. 6A shows a180 micron wide strip opened in 1000 A BSG (boron-doped oxide)/500 A USG(undoped oxide) for base isolation region; and

FIG. 6B shows a ˜90 micron wide stripe opened in 1000 A USG (undopedoxide) for base region;

FIG. 7A shows the threshold for oxide damage, below which metal can beremoved without metal penetration of the oxide layer;

FIG. 7B shows that after 20 scans the metal runners are fully isolated;

FIG. 7C shows an optical micrograph of the trench formed in this metalstack;

FIGS. 8A and 8B show a top view and a cross-sectional view of apyramidal TFSC;

FIGS. 9A and 9B show a top view and a cross-sectional view of a prismTFSC;

FIGS. 10A and 10B show a process flow for creation and release of aplanar epitaxial thin film silicon solar cell substrate (TFSS);

FIGS. 11A and 11B show a process flow for planar epitaxial thin filmsilicon solar cell substrate in case the TFSS is too thin to be freestanding or self-supporting;

FIGS. 12A and 12B show a process flow for micromold template (orreusable template) creation for making a 3-D TFSS;

FIGS. 12C and 12D show a process flow for 3-D TFSS creation using thereusable micromold template;

FIG. 13 shows a process flow for making a planar front contacted solarcell where the TFSS is thick enough to be free standing andself-supporting (e.g. thicker than approximately 50 microns for smaller100 mm×100 mm substrates and thicker than approximately 80 microns for156 mm×156 mm substrates), in accordance with the present disclosure;

FIG. 14 shows a process flow for making a planar front contact solarcell where the TFSS is too thin to be self supporting, in accordancewith the present disclosure;

FIG. 15 shows a process flow for making a 3-D front contact solar cellin accordance with the present disclosure;

FIGS. 16A-16D show a process flow for making an interdigitated backcontact back junction solar cell where the TFSS is thick enough to beself supporting, in accordance with the present disclosure;

FIG. 17 shows a process flow for making an interdigitated back-contactback-junction solar cell using thick TFSS where the in-situ emitter isnot deposited. Instead, a BSG (boron-doped oxide) layer is deposited onthe epitaxial silicon film and patterned to open the base isolationregion, in accordance with the present disclosure;

FIG. 18 shows a process flow for making an interdigitated back-contactback-junction solar cell where the TFSS is not thick enough to be selfsupporting, where in-situ emitter and laser ablation of silicon is usedto form the base isolation opening, in accordance with the presentdisclosure;

FIGS. 19A-19H show a process flow for making an interdigitatedback-contact back junction solar cell where the TFSS is not thick enoughto be self supporting, and where instead of in-situ emitter BSG(boron-doped oxide) deposition and selective laser etchback is used toform the base isolation opening, in accordance with the presentdisclosure;

FIG. 20 shows a process flow for making an interdigitated back-contactback-junction solar cell using a 3-D TFSS, in accordance with thepresent disclosure;

FIG. 21 shows a process flow for making an interdigitated back-contactback-junction hetero junction solar cell, in accordance with the presentdisclosure;

FIGS. 22 through 30 are not found in U.S. patent application Ser. No.13/118,295 “LASER PROCESSING FOR HIGH-EFFICIENCY THIN CRYSTALLINESILICON SOLAR CELL FABRICATION” by Virendra V. Rana and filed on May 27,2011;

FIGS. 22A and 22B are schematics showing the profile of a Gaussian beamand a flat top beam, respectively;

FIG. 23 is a cross-sectional diagram of a back-contact/back junctioncell;

FIGS. 24A-24F are rear/backside views of a back contact solar cellduring fabrication;

FIG. 25 is a rear/backside view of the back contact solar cell of FIG.24A with alternating metal lines contacting the emitter and baseregions;

FIGS. 26A-26C are diagrams illustrating three ways a flat-top beamprofile may be created;

FIGS. 27A and 27B are schematics showing the profile of a Gaussian beamand a flat top beam highlighting the ablation threshold;

FIGS. 28A and 28B are diagrams showing a Gaussian beam and a flat topbeam ablate region profile/footprint, respectively;

FIG. 28C is a graph of overlap and scan speed;

FIGS. 29A and 29B are diagrams illustrating a beam alignment window of aGaussian beam and flat top beam, respectively;

FIGS. 29C and 29D are diagrams showing a Gaussian beam region profileand a flat top beam region profile, respectively; and

FIG. 29E graphically depicts the results of Table 1;

FIG. 30 shows a process flow for an NBLAC cell;

FIG. 31 shows a schematic cross section of an NBLAC cell;

FIG. 32 shows a graph of minority carrier lifetime with and withoutlaser annealing;

FIGS. 33A and 33B show process flows for all back contact solar cellswith oxide ablation;

FIGS. 33C and 33D show process flows for all back contact solar cellswith oxide ablation;

FIGS. 34A and 34B show an oxide ablation process;

FIGS. 35A and 35B show an oxide ablation process using an amorphoussilicon layer;

FIG. 36, outlines the process flow to form back-junction, back-contactsolar cell from a starting wafer;

FIGS. 37A through 37E are scanning electron microscope (SEMS) imageshighlighting damage to an underlying silicon substrate during oxideablation;

FIG. 38 is a process flow for the formation of a back contact backjunction solar cell using a hard mask layer;

FIGS. 39A through 39I are diagrams of cell cross-sections schematicallyoutlining the solar cell structure corresponding to steps 2 through 8 ofthe process flow of FIG. 38;

FIG. 40 is a Minority Carrier Lifetime (MCL) map of a silicon substrateafter oxide ablation;

FIG. 41A is a scanning electron microscope (SEMS) image of overlappingablation spots;

FIG. 41B is an expanded view of the image of FIG. 41A;

FIG. 41C is a scanning electron microscope (SEMS) image ofnonoverlapping ablation spots;

FIG. 41D is an expanded view of the image of FIG. 41C;

FIGS. 42A and 42B are schematic diagrams showing two laser patterningopening and contact schemes;

FIGS. 43A and 43B are scanning electron micrographs of a spot-in-spotlaser pattern;

FIGS. 44A and 44B are cross-sectional diagrams of a solar cell having aninterdigitated orthogonal back contact metallization pattern;

FIGS. 45A and 45B are schematic diagrams outlining the self-alignedcontact ablation in accordance with the disclosed subject matter; and

FIG. 46 is a scanning electron micrographs of a contact openingself-aligned to the base opening.

DETAILED DESCRIPTION

Although the present disclosure is described with reference to specificembodiments, one skilled in the art could apply the principles discussedherein to other areas and/or embodiments without undue experimentation.

We disclose here laser processing, more specifically pulsed laserprocessing, schemes that have been developed to address the varyingrequirements of different processes.

The disclosed methods may be useful in the area of semiconductor deviceablation, particularly crystalline silicon ablation. Typically removalof silicon with a laser involves silicon melting and evaporation thatleaves undesirable residual damage in the silicon substrate. This damagecauses minority carrier lifetime degradation and increased surfacerecombination velocity (SRV) that reduces the solar cell efficiency.Hence, wet cleaning of the silicon substrate is typically used to removethis damage layer. We present a scheme to reduce this damage to a levelacceptable for high efficiency solar cell manufacturing that does notrequire post-laser-processing wet cleaning, hence simplifying theprocess flow and reducing the manufacturing cost.

The damage remaining in the silicon substrate upon ablating a certainthickness of it using a laser is related to the amount of laser energyabsorbed in the substrate that is not used by the ablated material. Ifit can be managed that most of the laser energy is used in removing thematerial then the fraction of the incident energy that seeps into thesilicon substrate is minimized, thus minimizing the laser-inducedsubstrate damage and SRV degradation. The penetration of laser energyinto silicon depends on the laser pulse length (also called pulse width)and wavelength. The infrared (IR) laser beam, wavelength 1.06 microns,has a long penetration depth in silicon, up to about 1000 microns, whilea green laser beam, with a wavelength of 532 nm, penetrates only to adepth of approximately 3 to 4 microns. The penetration of UV laser beam,with a wavelength of 355 nm, is even shorter, only about 10 nm. It isclear that using ultra-short pulses of UV or EUV wavelength limits thepenetration of the laser energy into silicon. Additionally, shorterlaser pulse length results in shorter diffusion of heat into silicon.While a nanoseconds pulse can lead to heat diffusion in silicon toapproximately 3 to 4 microns range, the picoseconds pulse reduces it toabout 80 to 100 nm, while a femtoseconds pulse is so short thattypically there is no heat diffusion into silicon during the laserablation process. Hence going to shorter pulses with a shorterwavelength lead to diminishing damage to the laser-ablated substrate.For higher production throughput, green or IR wavelengths can be useddepending on the extent of laser damage acceptable. Since even underideal conditions a certain fraction of the energy would still seep intothe substrate, this absorption and its undesirable side effects can befurther reduced by reducing the laser power. However, this results in asmaller thickness of silicon being ablated (or a lower silicon ablationrate or lower throughput). It has been found that reducing the pulseenergy but causing the silicon removal by increasing the overlap of thelaser pulses makes the silicon shallow isolation trench smoother. Thisis an indication of low silicon surface damage. At very low pulseenergies the thickness of silicon removed may be small. The desireddepth may then be obtained by using multiple overlapped scans of thepulsed laser beam.

A pulsed laser beam with pulse length in the picoseconds range and awavelength of approximately 355 nm or below is suitable for siliconablation with low damage enabling low surface recombination velocity(SRV) for passivated ablated surfaces. FIG. 1 shows a 2.25 micron deepand nearly 100 micron wide trench made in a silicon substrate using apicoseconds UV laser beam of Gaussian profile (M²≦1.3), nearly 110microns in diameter with 4 microjoule pulse energy, with the laser spotsoverlapped nearly 15 times. This depth of ablation was obtained usingtwenty overlapped scans of the laser with each scan removing about 112nm of silicon. FIG. 2 shows the smooth profile of a 4 micron deep and110 micron wide trench in silicon obtained using the same picosecondslaser beam with the UV wavelength. The smoothness of the profile shouldbe noted. Such an ablation of silicon is used in all back-contact backjunction solar cells to form regions that isolate base regions fromemitter regions. Use of a femtoseconds laser may provide furtherreduction of laser damage during silicon ablation.

The embodiments of this disclosure are also applicable to the ablationof amorphous silicon. A similar scheme may be used to ablate a desiredthickness of amorphous silicon using a pulsed laser beam withfemtoseconds pulse length and in some embodiments a UV or greenwavelength. Since ablation of amorphous silicon requires much lowerenergy than crystalline silicon, such a scheme may effectively be usedto selectively ablate amorphous silicon films from the crystallinesilicon surface for application to hetero junction solar cells.

This disclosure is also applicable to oxide ablation selective to theunderlying substrate, which may be crystalline or amorphous silicon. Theoxide film is transparent to laser beams of wavelength down to UV. If ananoseconds pulse length laser is used to remove the overlying oxide,the removal of oxide takes place by heating and melting of siliconunderneath. Because of the pressure from the ablated silicon underneath,the overlying oxide is cracked and removed. This however, creates heavydamage in the silicon substrate so that a wet cleaning treatment istypically used to remove this damaged layer for use in high efficiencycells.

We present here a scheme where the oxide layer is selectively removedfrom the silicon surface without any appreciable damage to the siliconsurface. During the laser ablation, besides heating the material to meltor evaporate it, other effects such as plasma formation take place.Sometimes complex processes can take place at an interface. Using alaser with picoseconds pulse length, the oxide to silicon interface isaffected. Using a picoseconds laser with a UV wavelength, the interfaceeffects are enhanced so that separation and delamination of the oxidefilm takes place from the silicon surface. The silicon surface leftbehind is virtually free of damage. Picoseconds laser radiation withgreen or infra-red (IR) wavelength can also be used depending on howmuch penetration damage of silicon substrate is acceptable. Thisdisclosure will outline the procedure to obtain damage free selectiveablation of oxide from the silicon surface.

FIGS. 3A-3D disclose the procedure for obtaining damage-free ablation ofoxide. FIG. 3A shows the variation of laser spot opening in a 1000 A PSG(phosphorus-doped oxide)/500 A USG (undoped oxide) stack on a 35 micronthick epitaxial silicon film on a template, using a picoseconds UV laserbeam. The oxide layers were deposited using APCVD (atmospheric-pressureCVD) technique. For a given thickness of oxide the spot size depends onthe laser fluence (J/cm²). The laser fluence is the laser pulse energydivided by the area of the laser beam. In this case, the laser beam wasabout 100 microns in diameter with a Gaussian profile (M²<1.3). At verylow fluence, the spots are irregular and there is irregular delaminationof oxide from the silicon surface as shown in FIG. 3B, while at veryhigh fluence there is extensive damage of silicon as shown in FIG. 3D.The range of fluence shown by line a-a′ indicates the optimum rangewhere the damage to the silicon substrate is minimal as seen in FIG. 3C.

FIG. 4 shows rows of cell contact openings that are selectively openedin the oxide for application in all back-contact (and back-junction)solar cells. FIG. 5 is a close-up of these contacts. The laser ablationspots can be overlapped in both x and y direction to open up an area ofany desired length and width on the wafer as shown in FIGS. 6A and 6B.FIG. 6A shows a 180 micron wide opening made by selectively removing theBSG (boron-doped oxide) for base isolation region using picoseconds UVlaser beam with ablation spots overlapping in both x and y-direction.Similarly, FIG. 6B shows a 90 micron wide area opened up in USG (undopedoxide) for forming the base region.

The selective ablation of oxide from a silicon surface as disclosed herecan be used in solar cell making in several ways. In one application,when using in-situ emitter for back-contact cells, this process is usedto open tracks in an oxide film to expose the underlying emitter. Theemitter so exposed may be removed using wet etching. This region is thenused for base to emitter isolation and with base formed inside it.

In another application, this process is used to open regions that arethen used for making metal contacts. For front contacted cells, theoxide passivation can be used on the backside of the cells. The schemedescribed here is then used to open contacts for the metal that issubsequently deposited on these contacts. In this manner, the metal haslocalized contact that is conducive to high cell efficiency. For backcontacted cells, contacts for both base and emitter may be opened usingthis scheme.

In a solar cell process flow, a doped oxide may need to be removedwithout causing any doping of the silicon underneath (i.e., without anyappreciable heating of the doped oxide and silicon structure). Since, asdescribed above, the oxide is removed by separation at the oxide/siliconsubstrate interface when using a picoseconds laser beam, the removal ofoxide happens with limited pickup of the dopant from the oxide filmbeing ablated.

The selective ablation of silicon nitride (SiN_(x)) is used for frontcontacted solar cells. Using laser ablation, the contact area to theemitter surface can be reduced thereby minimizing the area where the SiNpassivation is removed. This leads to higher V_(OC). Picosecond laserswith either UV or green wavelength are suitable for this application,although nanoseconds UV lasers can also be used.

Selective metal ablation from the oxide surface has historically beendifficult using lasers. This is because at the high pulse energiesneeded to ablate metal, the energy is high enough to damage the oxideunderneath and cause penetration of metal into oxide. In fact, this isthe basis for the process of “laser fired contacts” (LFC) used in solarcells.

We disclose three schemes for selectively removing metal from the oxide(or another dielectric) surface with no metal penetration of oxide (orother dielectrics such as silicon nitride) and breaking or cracking ofoxide. In all these schemes, aluminum is the first metal in contact withbase and emitter (aluminum being used as the contact and light trappingrear mirror layer). A laser with picoseconds pulse length is suitablefor this application. For high metal removal rate the IR wavelength isquite suitable. According to the first scheme, metal is ablated at apulse energy that is lower than the threshold for oxide ablation. If thethickness of metal removed in one scan is lower than the desiredthickness, multiple overlapping scans are used to remove the fullthickness of metal. Since the pulse energy is below the oxide ablationthreshold, a clean removal of metal from the oxide surface is obtained.However, the exact recipe used highly depends on the type of metal inthe stack, their thickness and surface roughness, etc.

FIGS. 7A-7C shows the ablation results when patterning a PVD-depositedbi-layer stack of 2400 A of NiV on 1200 A of Al on oxide. It is desiredthat the metal be removed completely between the runners withoutbreaking through the oxide layer underneath (to prevent shunts in thecell). FIG. 7A shows the threshold for pulse energy, below which thismetal stack can be removed without penetration of oxide. This threshold,besides depending on the metal stack characteristics described above,depends on the laser parameters such as spot overlap obtained using acertain pulse repetition rate of the laser as well as the scan speed.With increasing pulse overlap the threshold pulse energy would decrease,because of the energy accumulation in the metal. FIG. 7B shows thatusing a pulse energy below the threshold for oxide damage, more thantwenty scans provided complete isolation of metal runners as determinedby the 100M-ohm resistance between parallel lines. FIG. 7C shows a clean75 micron trench formed in the 2400 A NiV/1200 Al metal stack.

According to the second, high-throughput scheme higher pulse energiesare used, since a substantial part of the incident energy is absorbed asit is being ablated thereby reducing damage to the oxide. This approachmakes the laser ablation of metal a very high throughput process. Usingthis scheme we have ablated 1250 A Al/100-250 A of NiV, with or withouta tin (Sn) overlayer up to a thickness of 2500 A successfully using atwo step process. In the first step the softer metal is removed using 15microjoule pulses, followed by 30 microjoule pulses both overlappedfifteen times. For thicker aluminum such as 2000 A the second step canbe carried out at 50 microjoules with the same number of overlapping ofpulses.

The third scheme of metal ablation is applicable to highly reflectivefilms, for example Al/Ag stack (with Al in contact with the cell and Agon top of Al), such that most of the incident energy of the picosecondslaser is reflected and ablation is drastically reduced. In that case thesurface of the reflective metal (Ag) is first dented using a long pulselength nanoseconds laser, pulse length from 10 to 800 nanoseconds,followed by picoseconds cleanup of the aluminum underneath.

This disclosure is also applicable to the selective doping of asubstrate. For successful doping of silicon using an overlying layer ofthe dopant-containing material, the pulse energy should be high enoughto melt the silicon but not high enough to ablate it or the dopant layerabove it. As the silicon melts, the dopant is dissolved into it. Uponrecrystallization of this silicon layer, a doped layer is obtained. Forthis application a nanoseconds pulse length laser with green wavelengthis quite suitable because of its limited penetration into silicon.

The laser processing techniques described above are applicable to planarand 3-D thin-film crystalline silicon substrates. The laser processesdescribed here are suitable for any thickness of the silicon substrate.These include the current standard wafer thickness of ≧150 microns usedfor crystalline silicon solar cells. However, they become even moreadvantageous for thin, fragile wafers or substrates as the process incarried out without any contact with the substrate. These include thewafers thinner than 150 micron obtained from monocrystalline CZ ingotsor multi-crystalline bricks using advanced wire sawing techniques or byother techniques such as hydrogen implantation followed by annealing toseparate the desired thickness of wafer, or thin-film monocrystallinesubstrates (such as in the thickness range of from a few microns up to80 microns) obtained using epitaxial deposition of silicon on asacrificial separation/release layer such as porous silicon and itssubsequent lift off.

The laser processing is particularly suited to three dimensionalsubstrates obtained using pre-structuring of reusable templates andsilicon micromachining techniques. One such method is described in the'713 application (published as US2010/0304522). FIGS. 8A through 9B showthe 3-D thin film silicon substrates obtained using the techniquedescribed in that publication. FIG. 8A shows the top view while FIG. 8Bshows the cross-section of the TFSS so obtained. For pyramidalsubstrates, the tips may be flat or may end in a sharp point. FIGS. 9Aand 9B show the TFSS with prism structure obtained using a reusablepre-structured 3D template described in the reference above.

Although the laser processes and the process flows described here areapplicable to any thickness of the silicon substrate (from less than onemicron to over 100 microns), we disclose here their application to solarcells made using thin silicon substrates in the thickness range of fromless than 1 micron to about 80 microns, including but not limited tothose that are obtained using epitaxial silicon on porous silicon (orother sacrificial layer) surface of a reusable template as described inthe '713 application. To facilitate the understanding of ourapplication, the process flow for obtaining a desired thickness (e.g.from about less than 10 microns up to about 120 microns) of planarmonocrystalline TFSSs according to that publication is shown in FIGS.10A and 10B for planar TFSS that are typically greater than about 50microns so that they can be handled as self supporting substrates duringcell processing, and FIGS. 11A and 11B for planar TFSS that aretypically thinner than about 50 microns so that they are not selfsupporting during cell processing (and hence, are reinforced prior toseparation from their host templates). FIGS. 12A-12D show the processflow for obtaining three-dimensional pyramidal silicon substrates.Three-dimensional prism-shaped substrates can be obtained with similarprocesses, but using a lithography or screen printed pattern thatprovides for that structure.

The thin planar substrate obtained using the process flow of FIGS. 10Aand 10B may be processed according to the process flow of FIG. 13 toobtain high efficiency front contacted solar cells. It should be notedfor self-supporting TFSSs it is advantageous to process the templateside of the TFSS first before proceeding to the other side. Since thetemplate side of the TFSS is textured during the removal of thequasi-monocrystalline silicon remaining on the TFSS after its separationfrom the template it is preferably the frontside or sunnyside of thesolar cell. The laser processes of selective ablation of silicon oxideand silicon nitride (SiN) are used to advantage in making this frontcontacted solar cell.

FIG. 14 shows the application of various laser processes for making highefficiency front contacted solar cells using planar TFSSs where the TFSSis too thin to be free standing or self supporting during cellprocessing. It should be noted that in this case the non-template sidesurface is processed first with the TFSS on the template. Once thisprocessing is complete the TFSS is first attached to a reinforcementplate or sheet (also called a backplane) on the exposed processed sideand then separated from the template. After separation of thebackplane-attached (or backplane-laminated) thin-film crystallinesilicon solar cell, removal of residual porous silicon, texture etch,and SiN passivation/ARC deposition, and forming-gas anneal (FGA)operation processes are carried out on the released face of TFSS (whichwill end up being the front surface of the solar cell).

FIG. 15 shows the application of various laser processes for making highefficiency front contacted solar cells using 3-D front TFSS. For thisapplication it is advantageous to have pyramid tips on the template sidenot be sharp but end in flat ledges.

The processes described here are further uniquely suited to simplifyingthe all back-contact cell process flow.

FIGS. 16A-16D show the laser processes used on the planar epitaxialsubstrate to make a back-contact/back-junction solar cell where the TFSSis self supporting (i.e., no backplane attachment to the cell). In thisapplication the epitaxial emitter is deposited in-situ during siliconepitaxy following the deposition of the epitaxial silicon base. Theablation of silicon is then used to remove the emitter from the baseisolation regions. At the same time four fiducials are etched into oxideto align subsequent ablation to this pattern. Next, a thermal oxide isgrown to passivate the silicon surface that will become the back surfaceof the back-contact back junction solar cell. The epitaxial silicon filmis then disconnected or released from the template (by mechanicalrelease from the porous silicon interface). Next, the residual poroussilicon layer is wet etched and the surface is textured (both can bedone using an alkaline etch process). This will become the texturedfront surface or the sunnyside of the solar cell. Now, the thermal oxideis ablated using a picoseconds UV laser to form base openings inside thebase isolation region. The base opening is aligned inside the baseisolation region (trench) formed by silicon ablation earlier using thefiducials that were etched in silicon earlier as mentioned above. Next aphosphorous containing oxide layer (PSG) is blanket deposited on thesurface. Scanning with a nanosecond green or IR laser aligned to baseopening using the fiducials in silicon causes the base to be doped.Also, the region that will have the contact openings to emitter is alsodoped in a similar manner using the aligned scans of nanosecond green orIR laser. Next, contact opening are made to these doped base and emitterareas using a picoseconds UV laser. Again, the alignment of thesecontact openings is made using fiducials in silicon. Now, a metal stacklayer comprising aluminum as its first layer in contact with the cell(e.g., a stack of 1250 A Al/100-250 A NiV/2250 Sn) is deposited using asuitable method such as a PVD (physical vapor deposition) technique.Next, this layer is patterned using a picoseconds IR laser so that themetal runners are separately connected to the base and emitter regions.After an optional forming gas anneal (FGA), the cell is connected to andreinforced with a backplane with either embedded (Al or Cu)high-conductivity interconnects or no embedded interconnects (in thelatter, the final cell metallization can be formed by a copper platingprocess). The cell is now ready for test and use. FIG. 17 shows thelaser processes used on the planar epitaxial substrate to make aback-contact solar cell where epitaxial silicon base is not depositedwith an emitter layer. Instead, a boron containing oxide (BSG) layer isdeposited and patterned to open the base isolation region. A similarprocess to that described above is followed except that now the emitterand base are formed simultaneously during a thermal oxidation stepaccording to the process flow outlined in FIG. 17.

FIG. 18 shows a process flow using laser processes on the epitaxialsubstrate to make a planar back-contact/back-junction solar cell wherethe TFSS is not self-supporting (hence, a backplane is used). This flowuses the silicon ablation of in-situ doped emitter to form the baseisolation region.

FIG. 19A-19H show a process flow using laser processes on the epitaxialsubstrate to make a planar back contact solar cell where the TFSS is notself-supporting. In this flow, instead of an in-situ emitter layer, theBSG deposition and selective laser ablation followed by thermaloxidation (or a thermal anneal or a thermal oxidizing anneal) is used toform the emitter as well as the base isolation region.

FIG. 20 shows a process flow for making back contacted 3-D solar cells,it is advantageous to have the template side of pyramids end inrelatively sharp points. Since the 3-D TFSS can be self-supporting torelatively low thickness (e.g., silicon as thin as about 25 microns),the process flow is similar to that shown in FIG. 16. It should be clearthat we again have a choice of using the in-situ emitter followed bylaser ablation of silicon, or BSG deposition and selective laserablation followed by thermal oxidation (or thermal anneal, or thermaloxidizing anneal).

For applications in hetero junction solar cells, a hetero junctionemitter may be formed by a doped amorphous silicon layer in contact withan oppositely doped crystalline silicon base. For interdigitated backcontact solar cells we pattern the amorphous silicon layer and thetransparent conducting oxide (TCO) using laser ablation that isselective to the crystalline layer. Femtoseconds pulsewidth lasers witheither UV or green wavelength are suitable for this application. Aprocess flow is described in FIG. 21. Several variations of this processflow are possible.

Various embodiments and methods of this disclosure include at least onethe following aspects: the process to obtain silicon ablation ofcrystalline and amorphous silicon with reduced damage; the process toobtain oxide ablation for both doped and undoped oxides with no orreduced damage to silicon; the process to obtain fully isolated metalpatterns on a dielectric surface for solar cell metallization; theprocess to selectively dope the emitter and base contact regions; theuse of pulsed laser processing on very thin wafers, including planar and3-D silicon substrate; the use of pulsed laser processing on substratesobtained using epitaxial deposition on a reusable template made usingtemplate pre-structuring techniques; the use of various pulsed laserprocesses in making front contacted homo-junction solar cells; the useof various pulsed laser processes in making all-back contactedhomo-junction solar cells; and the use of various pulsed laser processesin making hetero junction solar cells.

Although the front contact solar cells are described with p-type baseand back-contact back junction solar cells are described with n-typebase, the laser processes described here are equally suited to thesubstrate with opposite doping, i.e., n-type for front contact solarcell with P⁺ emitter, and p-type base for back-contact back junctionsolar cells with p-type base and n⁺ emitter.

The following description, tables, and figures disclose the applicationof flat top laser beams to laser processing methods for interdigitatedback-contact cells (IBC). The description following is directed towardsmethods for the formation of back contact solar cells utilizing flat toplaser beams as compared to traditional Gaussian laser beams. Further,the implementation of flat top laser beams to the laser processingmethods described throughout this application provides substantialreduction in damage to silicon, improvement in solar cell fabricationthroughput, and a bigger alignment window for defining patterns (e.g.patterns of emitter and base regions) that are inset inside anotherpattern.

FIGS. 22A and 22B are schematics showing the profile of a Gaussian beam,FIG. 22A, and a flat top beam, FIG. 22B. The beam intensity of theGaussian beam has a smooth decrease from a maximum at the beam center tothe outside of the beam. In contrast, the intensity is “flat” or uniformfor the flat top beam through most of its profile (center to outside).

As disclosed herein, high-efficiency back-contacted, back junction cellswith interdigitated back contact (IBC) metallization benefits from theuse of at least one or several steps of pulsed laser processing. Laserprocessing may be utilized in several processing throughout theformation of the back contact cell, including: defining emitter and baseregions (or base-to-emitter isolation), defining back-surface field(BSF) regions, doping to form back surface fields, opening contacts inthe dielectric to base and emitter, and metal patterning. Some of thesesteps require laser processing of wide areas that are typically producedby overlapping Gaussian beam laser spots. Overlapping severely reducescell processing speed and may cause silicon damage, resulting indegradation of cell performance and yield. By replacing smaller diameterGaussian spots with a relatively wide flat top laser beam, substantialimprovement in throughput is obtained. And because the overlapping ofspots is dramatically reduced, the semiconductor (e.g., crystallinesilicon) substrate damage is reduced significantly. FIGS. 23-25illustrate embodiments of back contact solar cells that may be formedaccording to the disclosed flat top laser beam processing methods.

FIG. 23 is a cross-sectional diagram of a back-contact/back junctioncell with interdigitated back-contact (IBC) metallization formed from ann-type substrate, such as that disclosed herein. As shown in FIG. 23,alternating emitter and base regions are separated by relatively lightlyn-doped substrate regions (the n-type base). The rear/backside surfaceis covered by a surface passivation layer that provides good surfacepassivation with low back surface recombination velocity, made of, forexample: thermal silicon dioxide, deposited silicon dioxide, or siliconoxide/silicon nitride layers which may be deposited using techniquessuch as PECVD or APCVD (and/or aluminum oxide deposited by atomic layerdeposition or ALD). This surface passivation process may then befollowed by making openings in this passivation layer which act as‘localized contacts’ to the emitter and base regions. Then conductordeposition and patterning (e.g., aluminum as shown in FIG. 23) may beperformed to separately connect the emitter and base regions.

FIG. 24A is a rear/backside view of a back contact solar cellillustrating an interdigitated back contact base and emitter design withthe emitter and base regions laid out in alternating parallel rows. Thisbackside may be formed, for example, by starting with a surface that iscompletely covered by an emitter region, then delineating a base regionresulting in the formation of the patterned emitter regions. Then dopingbase contact regions with phosphorous is carried out and contacts areopened to the base and emitter regions in preparation for metallization.

FIGS. 24B-24F are rear/backside views of a back contact solar cellillustrating the back contact cell after key processing steps, whereinany one step or combination of steps may be performed according to alaser process which may or may not utilize a flat top beam. The variouslaser patterning steps of this particular exemplary method are outlinedin FIGS. 24B-24E. Starting with an n-type silicon substrate, a BSG layeris deposited over the whole surface. Next, the emitter to BSF isolationregion is defined using laser ablation of the BSG as shown in FIG. 24B.This step, the delineation of base and emitter regions, is referred toherein as the “BSG Opening” step. Alternatively, an in-situ boron dopedlayer may be deposited during silicon epitaxy and the BSF region definedusing laser ablation of silicon.

After the emitter to BSF isolation region is defined in the BSG Openstep, a USG layer is deposited on the wafer followed by laser ablationof this layer in patterns that are inlaid to the the BSG Open region, asshown in FIG. 24C. This patterning step is referred to herein as the BSFOpening step or base opening step. The BSF openings should be isolatedfrom the edges of the BSG Openings to prevent shunt formation as shuntsare deleterious to the solar cell efficiency.

Next, a PSG layer is deposited on the wafer and the silicon exposed toPSG in the BSF opening is doped using selective laser scans of thisarea. The doped BSF regions (base regions) are outlined in FIG. 24D

Next, the contacts to base and emitter are made using laser ablation asshown in FIG. 24E. It should be noted that the contacts may be pointcontacts as shown in FIG. 24E or line contacts as shown in FIG. 24F.Also, the number of contacts or the number of lines should be optimizedfor minimum series resistance of the current conduction path for thesolar cell—thus the designs and methods of the disclosed subject matterare not limited to the exemplary embodiments shown herein. It is alsoimportant that the contact openings are properly aligned inside theparticular doped area so that there is no current leakage.

As disclosed previously, a picoseconds pulse length laser may be usedfor oxide ablation processes of BSG open, BSF opening, and contactopening, although a nanoseconds pulse length laser may also be used.Further, although IR wavelength may be used, green or UV or smallerwavelengths are more suitable because of their reduced penetration intosilicon.

For BSF doping particularly, a nanoseconds pulse length laser may bemore suitable because of its penetration into silicon. And although IRwavelength may be used, green wavelength, because of its reducedpenetration compared to IR, may be more suitable for the depth of dopingtypically desired.

FIG. 25 is a rear/backside view of the back contact solar cell of FIG.24A with alternating metal lines contacting the emitter and baseregions. Note that the metal lines for the emitter and base regions areseparately connected to busbars not shown in FIG. 25 for simplicity ofthe figure. This metal pattern may be formed by blanket deposition of ametal followed by laser ablation of the metal to isolate base contactsfrom emitter contacts. Because relatively thick metal lines are requiredfor good current conduction (usually lines 20 μm thick or thicker), athinner metal stack such as aluminum/nickel-vanadium/Tin may be firstdeposited and patterned by lasers, followed by the selective depositionof a thicker metal such as copper using electro or electroless plating.Alternatively, a backplane with relatively thick conductors may beapplied and attached to the cell with thin conductor lines. Apicoseconds pulse length laser with IR wavelength may be most suitablefor ablating the metal stack with good selectivity to the underlyingoxide layer.

The disclosed flat top laser beam processing steps that may be utilizedto make this structure possible include, but are not limited to:delineation of emitter and base regions (BSF and emitter to BSFisolation) by laser ablation of an emitter or deposited boron dopingdielectric (such as boro-silicate glass BSG deposited by APCVD);delineation of the BSF region by opening the dielectric covering theopening made in the BSG; N+ doping of the base (e.g., with phosphorus);opening of metallization contacts to base and emitter regions; and metalpatterning using metal laser ablation to isolate base and emittercontacts. FIGS. 26A-26C are diagrams illustrating three ways a flat-topbeam profile may be created (diagrams reproduced from F. M. Dickey andS. C. Holswade, “Laser Beam Shaping: Theory and Techniques”, MercelDekker Inc., NY, which is hereby incorporated by reference in itsentirety). FIG. 26A illustrates one technique for creating a flat topbeam profile, the so-called “aperturing of the beam.” Using this method,the Gaussian beam is made flatter by expanding it and an aperture isused to select a reasonable flat portion of the beam and to cut-out thegradually decreasing ‘sidewall’ areas of the beam. Using this method,however, may cause a significant loss of beam power.

A second example method for creating a flat top beam, as shown FIG. 26B,uses beam integration wherein multiple-aperture optical elements, suchas a micro-lens array, break the beam into many smaller beams andrecombine them at a fixed plane. This beam integration method may workvery well with beams of high M² value.

A third beam shaping system for creating a flat top beam, as shown FIG.26C, uses a diffractive grating or a refractive lens to redistribute theenergy and map it to the output plane. Any known method, including thethree example techniques disclosed in FIGS. 26A-26C, may be used obtainthe flat top beam profile for applications described herein. Thesuitability and choice of a flat top laser beam formation method dependson a variety of factors including the available beam characteristics andthe results desired.

FIGS. 27A and 27B are schematics showing the profile of a Gaussian beamand a flat top beam highlighting the ablation threshold. As shown inFIGS. 27A and 27B, a flat top laser beam, particularly as compared to aGaussian beam, can substantially reduce the laser damage during ablationand doping processing. For Gaussian beams there is substantial excessivelaser intensity above that required for ablation, particularly in thecenter of the beam, that can cause damage of silicon (as shown in FIG.27A). The flat top beam can be configured so the peak intensity is onlyslightly above that required to ablate the material (the ablationthreshold as shown in FIG. 27B) and the damage that may be caused by thehigh intensity of the Gaussian beam is avoided.

A flat top beam, whether having a square or rectangular cross section,offers throughput advantages particularly as compared to a Gaussianbeam. FIG. 28A is diagram showing a Gaussian beam ablated regionprofile/footprint. The circular shaped spots of a Gaussian beam arerequired to overlap substantially to the minimize the zigzag outline ofthe pattern, typically as much as 50% overlap (FIG. 28A). FIG. 28B isdiagram showing a flat top beam ablate region profile/footprint. Sincethe square or rectangular flat top beam have flat edges, thus creating aflat outline, the overlap can be significantly reduced (FIG. 28B). FIG.28C is a graph showing the improvement in scan speed as beam overlap isreduced. Note that even for an overlap of 30%, a scan speed increase of33% may be realized.

FIG. 29A is a diagram illustrating a beam alignment window of a Gaussianbeam and FIG. 29B is a diagram illustrating a beam alignment window of aflat top beam. As can be seen in FIGS. 29A and 29B, yet anotheradvantage of using a flat top beam for making inlaid patterns is thelarger alignment window the flap top beam provides. The circular shapedspots obtained from a Gaussian beam create zigzag edges of the ablatedregions (FIG. 29A). The alignment margin of M as shown in FIG. 29A isreduced and limited to M-a-b due to the waviness of the zigzag edgeprofile.

However, the ablation region edges created using a flat top beam arestraight allowing the alignment margin to stay at M. For the backcontact back junction solar cells described herein, BSF openings areformed inside the BSG Open regions, and contact openings are formedinside the BSF region. Hence, a larger alignment margin is important asit allows for smaller BGS Open, BSF, and contact regions. Thus reducingthe electrical shading and improving solar cell performance.

Since the overlap of square or rectangular flat top beam can be reducedin both x and y direction while making a large area ablation or doping,the throughput is significantly enhanced. Also, since the size of thesquare or rectangular flat top can be increased without causingexcessive zigzagging of the perimeter, throughput is further increased.Table 1 shows the reduction in the number of scans needed to open a 150um wide line, such as used for delineating the base area by ablating theBSG film.

Table 1 below shows the throughput of Gaussian vs. Flat Top laser beamsfor creating a 90 μm wide base opening. The results of Table 1 are showngraphically in FIG. 29E.

TABLE 1 Width Spot Pitch Number of line Size Overlap of scans of scansPROCESS (um) (um) % (um) per line BSG Ablation with 150 30 50 15 9Gaussian BSG Ablation with 150 30 20 24 6 Flat Top BSG Ablation with 15060 20 48 3 Flat Top

FIG. 29E shows the throughput advantage of flat top beams (the 60 μmflat top beam region profile is depicted in FIG. 29D) as compared to theGaussian beam (the 30 μm flat top beam region profile is depicted inFIG. 29C), for a high productivity laser system that can process fourwafers at a time. To further reduce cost, for example, two lasers may beutilized with each laser beam further split into two. However, manyvariations of this flat top laser beam hardware and fabrication schemeare possible.

Also, because overlap is significantly reduced in both x and ydirections when using a flat top beam, the laser induced damage ofsilicon is greatly reduced as compared to the Gaussian beam.

Similar throughput advantages may also result when utilizing a flat topbeam for opening the oxide region for BSF, doping the BSF region usingthe overlying PSG, forming base and metal contact openings if they areline contacts, and the metal ablation isolation lines—all with theconcurrent advantage of reduced silicon damage. Additionally, utilizinga flat top beam provides the advantage of increased alignment window forBSF opening inside the BSG opening and contact opening inside the BSF.Flat top laser processing methods may also increase throughput forforming a back surface field. For example, the back surface field may beformed by doping the base region, opened as described, with an n-typedopant such as phosphorous. For this process the base is covered with aphosphorus-doped silicon oxide (PSG) layer and the doping may beperformed by irradiating this region with a laser beam. While uniformlydoping this region using Gaussian laser beams requires overlapping,overlapping is minimized or may be completely reduced using a flat topbeam. And as with the base and emitter region delineation and backsurface field delineation described herein, utilizing a flat top laserbeam provides a substantial throughput and reduced damage advantage asrequired overlapping is decreased. It should be noted that for forming aback surface field, the beam need to be flat top beam only in onedirection—normal to the scan, whereas it may be Gaussian in thedirection of the scan. This type of beam is called a hybrid flat topbeam.

Importantly, for forming isolated base or emitter contacts, althoughoverlap is not an issue, the silicon damage is still reduced using aflat top beam because of the absence, unlike Gaussian, of a highintensity peak in the center of the beam (as shown in FIGS. 27A and27B).

Another aspect of this disclosure relates to the use of laser annealingto improve the conversion efficiency performance of crystallinesemiconductor solar cells in general, and crystalline silicon solarcells in particular, by improving the passivation properties ofdielectric-coated surfaces, and more specifically silicon nitride(SiN)-coated surfaces. The improved front surface passivation propertiesare manifested as reduced Front-Surface Recombination Velocity (orreduced FSRV) and increased effective minority carrier lifetime. Thistechnique is especially advantageous for high-efficiency back-junction,back-contacted cells with interdigitated metallization (IBC) whereannealing of SiN-coated front surface may also be used to concurrentlyresult in the annealing of emitter and base metal contacts on the solarcell back surface, thereby, lowering the specific contact resistivityand improving the solar cell fill factor (FF). The laser annealingmethods of this disclosure are applicable to crystalline semiconductorsolar cells using semiconductor absorber layers over a wide range ofthicknesses, i.e., thick wafer-based solar cells such as crystallinesilicon wafer solar cells with wafer thicknesses of 10's to 100's ofmicrons. Moreover and more specifically, the non-contact laser annealingprocess and methods of this disclosure are applicable to extremely thin(e.g., crystalline semiconductor layers from a few microns to ˜50microns thick) crystalline silicon solar cells where unsupported cellmechanical handling can result in cell breakage. It is also an in-linereplacement for the batch furnace annealing processes. The laserannealing process and methods can be used as the last step in the cellmanufacturing process flow or immediately after deposition of thefront-surface passivation and anti-reflection coating (ARC) layer. Theprocesses and methods of this disclosure enable formation ofhigh-quality surface passivation and ARC layers using low-temperature,low-thermal budget deposition processes for passivation & ARC layerssuch as silicon nitride deposited by low-temperature PECVD.

The passivation of the surface of phosphorous-rich N⁺ emitter withsilicon nitride for standard front contact solar cells with p-typesilicon bulk (or p-type base), is well known and widely utilized in thesolar industry. While the SiN film acts as an antireflection coating toreduce the optical reflection losses and to increase sunlight trapping,it also serves a very important task of passivating the surface of thephosphorous-rich N⁺ emitter by the well-known hydrogenation process. Thehydrogen released from the hydrogen-containing SiN layer satisfies theopen bond on the silicon surface (or silicon dangling bonds causingsurface states and traps), thereby reducing the surface recombinationvelocity or rate of minority carriers by these dangling bond sites. Forcells made from multi-crystalline or polycrystalline silicon, thishydrogen provided by the SiN layer further reacts with the impuritiesand defects in the bulk of the silicon wafer as well as removes thegrain boundary trap sites, thereby reducing the overall minority carrierrecombination and increasing the effective minority carrier lifetime inthe bulk of the material.

The release of hydrogen and hence the surface and bulk passivation ofsilicon is typically obtained during the so-called “metal firing”process in the standard front-junction/front-contact solar cellmanufacturing process flow, currently widely used in the solar cellmanufacturing industry. The screen-printed metal firing process consistsof multiple-step heating of the solar cell using a carefully designedtemperature and time sequence with a final dwell at about 850-900° C.before a desired cooling sequence. This firing cycle is optimized aftercareful experimentation. Since hydrogen is a small atom it can diffuseout of the wafer if the wafer temperature is too high or the annealingtimes are too long. On the other hand, the hydrogen passivation may beunsatisfactory if the temperature is too low or annealing times are tooshort. Hence, the hydrogen-passivation phenomenon has been a subject ofintense investigation and research in the solar cell industry and isconsidered not just science but also an art by many (since there arestill many areas yet to be fully understood). It is clear that a processthat can provide a high degree of control is thus desired.

For the standard mainstream front-contact solar cell with p-type siliconbulk (or p-type boron-doped base) and n⁺ phosphorus-doped emitter, thefront contact surface is contacted by silver while the back surface iscontacted by aluminum—which may be screen printed as a blanket layer ormake selective contacts through openings made in the backside dielectricsurface. To obtain low resistance contacts, the intermixing of silverwith silicon in the front and aluminum in the back is promoted duringthe metal firing process that has been described above. Based on thedescription of the metal firing process above, the practice of obtaininglow resistance contacts and hence high FF in the solar cell iscomplicated. Again, a process that can provide a high degree of controlis desired.

Additionally, the all back-contact, back junction solar cells that usethe same metal, aluminum, in contact with both n⁺ and p⁺ contacts on theback side cannot be heated too high as the doping of n⁺ contact byaluminum, a p-type dopant, will increase the contact resistance, therebylowering the fill factor of the cell. Moreover, overheating of aluminummuch above 450° C. can result in degradation of optical reflectance ofaluminum (and thus increased optical losses of the infrared photons inthe cell). A controlled low-temperature heating, preferably in the rangeof 200-450° C., of the contacts where aluminum makes intimate contactwith silicon by reducing and absorbing the oxide at the silicon surface,is highly desirable.

We disclose here a process where the front surface or sunnyside of thesolar cell is substantially uniformly or in selected areas irradiatedwith the laser beam, selectively heating the semiconductor (e.g.,silicon) such that hydrogen atoms are released from SiN therebyeffectively passivating the silicon surface, reducing the surface statedensity, reducing the front-surface recombination velocity (FSRV), andincreasing the effective minority carrier lifetime of the solar cell.The processes and methods of this disclosure may also reduce the bulktrap density and enhance the bulk minority carrier lifetime. Oneembodiment of the disclosed method is based on using a pulsed lasersource with a wavelength smaller than that of the semiconductor (e.g.,silicon) bandgap. In this embodiment (for instance, using a pulsed greenor UV laser source for crystalline silicon surface annealing), thefront-surface is selectively heated using pulsed laser sourceirradiation, while the backside of the cell remains substantially coolerthan the frontside of the cell. Another embodiment of the disclosedmethod is based on using a pulsed laser source with a wavelength near toor larger than that of the semiconductor bandgap. In this embodiment(for instance, using a pulsed IR laser source for crystalline siliconsurface annealing) while the front-surface is heated using pulsed lasersource irradiation, the backside of the cell is also heated andannealed. Using this alternative embodiment, at the same time the laserbeam penetrates to the back of the solar cell heating the Al/siliconcontacts to decrease the contact resistance and to improve the overallcell fill factor and efficiency. The laser annealing process and methodsof this disclosure may be performed at the end of the solar cellfabrication process flow or immediately after formation of thepassivation/ARC layer and before the cells are tested and sorted formodule packaging. Alternatively, the laser annealing process and methodsof this disclosure may be performed after assembling and packaging thecells in a PV module and through the front glass cover of the moduleassembly. In this case wavelengths need to be used that can go throughthe glass, such as infrared.

It is important that the laser anneal process should be optimized(including the laser source wavelength, pulse width, power, etc.) suchthat the passivation layer (e.g., the PECVD SiN layer) is not degradedduring this process so that the sunlight can pass through thisantireflection coating without significant optical absorption losses.Also, the surface texture should not be affected so that the lighttrapping is not reduced. It is clear that the type of the pulsed lasersource and the laser process parameters should be carefully chosen tomeet all these requirements.

The laser pulse length should be long enough so that there is nonon-linear optical interaction with the passivation/ARC layer (e.g., SiNlayer) so that the passivation/ARC layer) is unaffected. Although,lasers with pulse length from 1 nanosecond to microseconds or continuouswave can be used for this application, the choice depends on the depthto which the heat penetration is desired. Using shorter pulse length theheat is limited to shallow depths. Wavelength also should be chosenbased upon the depth of semiconductor (e.g., crystalline silicon) thatis required to be heated. For applications to single crystal solar cellswhere only front surface passivation is required to be improved, greenwavelength may be more suitable. For applications where improved bulksilicon passivation is required and/or back contact annealing isdesired, IR wavelength may be more suited. It should be clear that basedon the desired application a range of laser pulse length and wavelengthscan be used.

Processes for back contacted cells with interdigitated metallization,called NBLAC cells, have been described in related applications (see,e.g., U.S. patent application Ser. No. 13/057,104).

FIG. 30 outlines one of the embodiments of the NBLAC process flow, whileFIG. 31 is the schematic of the cross section of the cell (the backplaneis not shown for clarity). The low-temperature front-surfacepassivation/ARC: PECVD (silicon nitride)+laser anneal process step inFIG. 30 involves the deposition of SiN at lower temperatures than isused in the industry (<350 C). The surface is then subjected to pulsedlaser irradiation causing preferential silicon frontside annealing thatresults in improved passivation of the silicon surface with hydrogenfrom the SiN. In particular, the laser annealing processes and methodsof this invention enable formation of high-quality passivation and ARClayers (like single layer SiN and bilayer SiN with amorphous silicon)deposited at low temperature as low as 90° C., and more typically in thedeposition temperature range of 90° C. to 250° C.

In some embodiments, the SiN being annealed may contain a desired amountof phosphorus dopant. In this case, the annealing step also causessilicon doping with phosphorus. This process is discussed in connectionwith FIG. 36 below.

Besides SiN, silicon oxynitride (Si_(x)O_(y)Nz), or silicon carbide(Si_(x)C_(y)) single layers or a bilayer stack with SiN on amorphoussilicon (α-Si), a bilayer stack with SiN on silicon oxide (SiO₂), or abilayer stack with SiN on silicon oxynitride, can also be used forsilicon surface passivation. For example, it is known that an amorphoussilicon layer can passivate the silicon surface quite well. However, forthe current industrial process, significant surface cleaning of siliconand process optimization of the α-Si deposition process is required.Laser annealing of α-Si films covered with hydrogenated SiN can activatethe hydrogen in SiN and lead to dramatic enhancement of passivation, asmeasured by substantially increased effective minority carrier lifetimeand substantially reduced front-surface recombination velocity.

The PVD Al/NiV/Sn contact & backside reinforcement BSR step and thepulsed picosecond laser ablation of Al for interdigitated cell base &emitter Al lines step in FIG. 30 form the metal contacts to the base andemitter on the back surface of the solar cell. These contacts are shownin the cross section in FIG. 31. It should clear that the laser beamthat penetrates to the back of the silicon film will concurrently annealthe back contacts, resulting in reduced contact resistance and increasedfill factor of the solar cell.

Results obtained using laser annealing are shown in FIG. 32. It is seenthat up to 100 times effective lifetime improvement is obtained onlow-temperature-deposited passivation layer of SiN without resorting tohigh temperature metal firing. In the NBLAC process the thin epitaxialsilicon is supported on a backplane. In case this backplane cannotwithstand a high temperature, the SiN deposition temperature is reducedto facilitate thin epitaxial/backplane assembly processing and processintegration accommodating the heat sensitive backplane assembly. Forsuch heat sensitive backplanes the laser annealing is highly suitablesince with a suitable selection of laser pulse length and wavelength,the heat can be limited to the front side of the silicon while keepingthe backside of the silicon within the acceptable value for thebackplane.

The non-contact laser annealing process is highly suitable for NBLACcells that use epitaxial films having thickness approximately in therange of a few to 50 microns, which are fragile to handle.

For enhanced throughput and improved process control, the laser sourceused for these applications may have top-hat profile (with relativelyuniform beam power over at least 100 micron or more) in order to reducethe overall surface irradiation scan time. This also eliminates thechance of damage in beam overlapping areas.

This laser annealing process is an attractive alternative to furnaceannealing as it can be an in-line cost effective process.

According to another aspect of the present disclosure, the selectivelaser ablation and patterning of electrically insulating layers, such asthermally grown or chemical-vapor-deposited silicon oxide on silicon isused in crystalline silicon solar cell process flows for obtainingrelatively high cell efficiency values. In such applications it isadvantageous that no or at most negligible damage is introduced in theunderlying silicon substrate, since any substantial ablation-induceddamage can lead to increased minority carrier recombination loss,resulting in further loss of cell conversion efficiency. We present herea novel scheme that ensures that the solar cell semiconductor (e.g.,silicon) surface will not be damaged during the pattern-selectiveablation of the dielectric (e.g., silicon oxide) overlayers. Thisdisclosure involves introducing a thin intermediate layer of siliconthat stops the laser beam from reaching the silicon substrate. This thinintermediate silicon layer may be placed closer to the underlyingsilicon surface, separated only with a thin buffer layer of siliconoxide. The layer of oxide above this intermediate silicon layer isablated by the laser beam interacting and separating the siliconoxide-intermediate silicon layer interface. A very thin (for example, 3nm to 100 nm or in some embodiments 3 to 30 nm) layer of silicon oxideunder this intermediate silicon layer prevents any significantdamage-causing effect of laser action at this interface from reachingthe silicon substrate. The intermediate silicon layer is subsequentlyoxidized (using either a thermal oxidation process or an oxidizinganneal process), thereby eliminating any unwanted interaction insubsequent laser processing. This scheme is particularly suited forapplication in an all-back-contact back junction solar cell design wherelaser ablation of dielectric layers such as silicon oxide is utilizedseveral times, such as the NBLAC solar cell.

In one embodiment of a process flow, the oxide ablation process is usedthree times to form oxide patterns, namely BSG (or BSG/USG stack)ablation to delineate emitter and base regions, USG (or PSG/USG stack)ablation to define the base regions, and finally ablation of PSG(phosphosilicate glass-oxide) to open contacts to base and the ablationof BSG/USG/PSG ablation to open contacts to the emitter regions. Thetechnique described herein can be advantageously used in the first stepof ablation of the BSG layer to define the patterned emitter and baseregions (for solar cells using n-type base). If desired, this techniquecan be further used during the ablation of USG for defining the openingsfor N⁺ base regions. (These polarities would be reversed for solar cellsusing p-type base.)

FIG. 33A shows a process flow for an all back contact solar cell thatinvolves oxide ablation at three different steps. FIG. 33B shows theslight modification to the BSG/USG (USG is undoped silicate glass orundoped silicon oxide) deposition step where a very thin α-Si layer isdeposited on top of a thin USG layer (in some embodiments in situ withinthe same APCVD BSG deposition equipment) before the deposition of theremaining BSG/USG stack. During the laser ablation process, the laserbeam separates the BSG/α-Si interface, thereby removing the BSG/USGstack. This thin layer of silicon is oxidized during the subsequentsteps as described in FIG. 33B.

FIGS. 33C and 33D show a further modification to the process flow ofFIGS. 33A and 33B, where the USG deposition step is modified to includethe deposition of the very thin α-Si layer on top of a very thin USGlayer before the deposition of the thicker USG layer. During the laserablation the laser beam separates the top USG/α-Si layer, therebyremoving the top USG layer. As before, this thin layer of silicon isoxidized along with the previously deposited α-Si as described aboveduring the subsequent step as shown in FIG. 33D.

FIG. 34 shows schematically a standard oxide ablation process using alaser beam with pulse width in the range of a few picoseconds. It can beseen that the interface being acted upon by the laser is the surface ofthe silicon substrate that may be damaged if the correct pulse energy isnot used. FIG. 35 shows the scheme where a very thin amorphous siliconlayer is deposited after the deposition of a very thin USG layer. Asshown in FIG. 35B, the interface for laser action is the BSG/amorphoussilicon interface. This interface acts as an ablation stopping layer andshields the crystalline silicon surface from laser irradiation therebypreventing or suppressing any possible crystalline silicon surfacedamage, resulting in higher cell efficiency.

The complete stack USG/α-Si/BSG/USG may be deposited in situ using APCVDfor solar cell fabrication. The APCVD equipment may be high-productivityin-line APCVD equipment with multiple sequential in-line depositionzones to enable deposition of the entire stack in a single piece ofAPCVD equipment. Using APCVD equipment, the thin undoped silicon layermay be deposited in one of the APCVD deposition zones (the second zoneafter deposition of the initial USG layer) using e.g. silane and argon(or silane and nitrogen) at a temperature of less than approximately500° C. Alternatively, it can be deposited using a PECVD technique. Awide range of thicknesses of thin USG and thin α-Si can be used basedupon the particular process flow. Typically, the USG in contact with thecrystalline silicon surface may be in the range of 3 nm to 100 nm, whilethe amorphous silicon layer may be in the range of 3 nm to 30 nm.However, as mentioned above, thicknesses outside of these ranges willalso work if the rest of the process flow is changed to accommodate thethickness of these films.

The same scheme can also be used, if so desired, to open oxide layer forbase regions that will be subjected to phosphorous doping to form N⁺layer. In that case the process flow is modified to ensure oxidation ofthis α-Si layer.

Various aspects of the laser processing innovations and correspondingsemiconductor, passivation, doping, metallization materials disclosedherein may be used singularly or in combination to improve solar cellefficiency. The following laser patterning methods utilize a laserabsorbent hard mask (e.g., amorphous silicon) in combination with wetetching to form patterned solar cell doped regions which may furtherimprove cell efficiency by completely avoiding laser ablation of thesemiconductor substrate associated with ablation of an overlyingpassivation/oxide layer. Further, the passivation materials andassociated laser oxide ablation methods as well as laser dopingparameters disclosed above may be used in conjunction with the hard maskmethod described for solar cell efficiency improvement.

High efficiency back-junction, back contact solar cells withinterdigitated metallization formed over alternating base and emitterregions (e.g., and interdigitated back contact IBS solar cell) oftenrequire very fine patterning of passivation layers to obtain high solarcell efficiency. Laser ablation of these layers may be used to obtainsmall size pattern dimensions. However, the passivation layers oftenused (e.g., patterned silicon oxide, aluminum oxide, etc.) aretransparent to laser wavelengths down to ultraviolet (UV). Thus, it isnot possible to selectively ablate these layers without damaging theunderlying silicon substrate to some degree. The solution disclosedherein provides a damage-free laser patterning method which avoidsablation of an underlying substrate altogether. In one scheme anabsorbent layer is used as a mask to be patterned by laser followed bywet etch of the transparent passivation layer. And although thesolutions provided are described with reference to an amorphous siliconabsorbent mask layer, other materials such silicon carbide, carbon richamorphous silicon or a non-conducting ceramic.

Importantly, the hard mask innovations disclosed herein are described inthe context of interdigitated back contact solar cells but are alsoapplicable to other solar cell structures such as heterojunction,PERL/PERC solar cells, or gallium arsenide based solar cells. Generalhardmask material requirements for damage-free laser ablation patterningwith hardmask plus wet etch are provided in Table 1 below which may beused as a damage-free laser ablation patterning solution for varioustypes of semiconductor devices.

TABLE 1 Hardmask material requirements for damage-free laser ablationpatterning with hardmask plus wet etch. IBC solar cell Stepapplication 1) Start with a semiconductor device substrate silicon solarcell 2) Deposit thin-film #1: dielectric layer or Boron-doped siliconsemiconductor layer, e.g. for passivation and/ oxide or boron-doped orelectrical isolation and/or dopant source. aluminum oxide 3) Depositthin-film #2: dielectric or Amorphous silicon, semiconductor layer forhardmask, with or amorphous silicon following requirements: carbide(higher carbon a) Low laser ablation threshold/energy content may enablelower compared to thin-film #1 laser ablation threshold) b) Low wet-etchrate compared to thin-film #1 c) no pinholes 4) Laser ablation (withlaser wavelength, pulse Picosecond, UV, or flat time, energy) toselectively pattern thin film #2 top nanosecond UV 5) Wet etch toselectively remove thin-film #1 Using an HF solution such as 100:1 HF,50:1 HF, or 10:1 HF, depending on the control of etch time desired

Importantly, the disclosed hard mask layer may be a laser absorbingmaterial such as silicon carbide or amorphous silicon deposited usingmethods such as atmospheric pressure chemical vapor deposition APCVD(e.g., using a disilane source gas), plasma enhanced chemical vapordeposition PECVD, or physical sputtering. The disclosed hard mask layermay be also be a laser absorbing nonconductive ceramic such as aluminumnitride (AlN), or a stack of amorphous silicon and silicon nitride (SiN)or amorphous silicon and AlN or any combination thereof. The passivationlayer may be a transparent dielectric material such as an oxide (e.g.,APCVD or thermal deposited oxide) a silicon oxynitride (e.g., a stack ofSiO2/SiOxNy), aluminum oxide (e.g., APCVD or ALD deposited aluminumoxide), or a nitride such as silicon nitride.

Aspects of the process flow disclosed in FIG. 30 describe the laserpatterning of passivation layers to form high efficiency threedimensional solar cells using epitaxially deposited thin crystallinesilicon films. These same laser patterning schemes are applicable tostandard back-junction, back-contact solar cells using planarcrystalline silicon wafers. FIG. 36, outlines the process flow to formback-junction, back-contact solar cell using laser ablation of oxideusing a picoseconds laser with UV (355 nm) wavelength to form selectiveemitter, base and contact openings. It should be noted that the use of abackplane to support the silicon film may make this process flowsuitable for thin silicon films having a thickness as small as 10 um.

Passivation layers such as silicon oxide and aluminum oxide aretypically transparent to wavelengths as short as 355 nm (UV). To somedegree laser beams with wavelengths down to UV pass through thesepassivation layers and attack and damage the silicon substrate. Thisdamage may be mitigated and reduced, for example using the methods andstructures outlined above, to have minimal effect on solar cellefficiency. For example, using shorter wavelengths reduces thepenetration of the laser beam into silicon. And shorter pulse laserbeams limit the heat penetration into silicon. Thus, it is advantageousto go to shorter wavelengths and shorter pulse lengths for the ablationof these transparent layers—a substantially reduced heat affected zonemay be obtained when going from, for example, 1064 nm (IR) to 355 nm(UV), and going form nanoseconds laser pulse to picoseconds andfemtoseconds laser beams.

These solutions may lead to reduced silicon damage when ablating anoverlying oxide layer (i.e., reducing damage to a negligible impact oncell efficiency). FIGS. 37A through 37E are scanning electron microscope(SEMS) images highlighting damage to an underlying silicon substrateduring oxide ablation—specifically laser damage found when ablatingusing a Gaussian laser beam having an approximately 10 picoseconds pulsewidth and 355 nm wavelength. FIG. 37A, is a SEMS image of an ablationspot in Si formed using a laser at a high laser fluence. It may be notedthere is extensive damage in the center of the spot due to the highpower at the Gaussian peak. Additionally, there are ripples extendingtowards the ablation edge. It is clear that this damage can be reducedby lowering the laser fluence to the minimum required. FIG. 37B, is aSEMS image of an ablation spot in Si formed using a laser at anoptimized laser fluence. The ripple damage is substantially reducedhowever minimal ripples may still be observed in the ablated spots.FIGS. 37C and 37D are two SEMS images showing a magnified view ofripples and silicon melting near the ablation spot in Si formed using alaser at an optimized laser fluence (e.g., that shown in FIG. 37B).Droplets of silicon may be observed near the oxide ablation edge inFIGS. 37C and 37D. FIG. 37E is a Transmission Electron Micrograph of theablation edge showing the creation of amorphous silicon by thepicoseconds UV laser ablation of an overlying oxide. Amorphous siliconformation may be observed in the open spot as well as some distanceunder the oxide that is present outside the ablation area. Thisamorphous silicon is typically seen in the optical microscope as a grayhalo around the ablation spot. As silicon oxide is transparent to thepicoseconds UV beam, oxide ablation occurs due to the melting andsubsequent evaporation of molten silicon. Any silicon not able to escape(e.g., at the ablation edge or the ripples seen in FIG. 37) solidifiesas amorphous silicon because of the extremely rapid heating and coolingrates associated with ultrafast picoseconds laser beam. Although, thisamorphous silicon may crystallize during subsequent process steps, forexample during furnace annealing such as that shown in FIG. 36, thepassivation of the silicon/oxide interface may be degraded which maycause minority carrier recombination at these sites leading to loweredsolar cell efficiency. And while this damage may be reduced andminimized to negligible impact utilizing the methods herein, in someinstances it may be desired to avoid this damage altogether.

The presently claimed subject matter provides a laser absorbent layerwhich may be laser patterned to act as a hard mask for subsequent wetetching of an underlying oxide. FIG. 38 is a process flow for theformation of a back contact back junction solar cell using an amorphoussilicon hard mask layer. Modified/added process steps in FIG. 38 ascompared to FIG. 36 are highlighted for comparison. In other words, FIG.38 shows a process flow where a thin layer of amorphous silicon is usedto absorb the laser beam in the process flow of FIG. 36. Regions soopened (i.e., the amorphous silicon layer ablation pattern) are thenfurther opened to underlying silicon by wet etching the oxide (BSG 1/USGin this case). Essentially, the amorphous silicon (α-Si) is used as amask that is patterned by the laser and the patterned is thentransferred to silicon by wet etching. The absorbent amorphous siliconprotects the underlying silicon substrate during laser ablation. Thus,there is no laser attack of the underlying silicon and no laser damageand the silicon minority carrier lifetimes are not affected. Hence, thelaser damage, a serious impediment to obtaining high solar cellefficiency, is avoided and eliminated.

Importantly, the buried amorphous silicon hard mask is buffered from thesilicon interface by an oxide layer, and subsequent processing such asannealing further neutralizes the buried amorphous silicon hardmask—i.e., acting as a benign layer.

FIGS. 39A through 39I are diagrams of cell cross-sections schematicallyoutlining the solar cell structure in process steps where amorphoussilicon (α-Si) is used as the patterning mask for laser ablationcorresponding to the process flow of FIG. 38. The followingcross-sections of FIG. 39 correspond to the following steps in FIG. 38(steps 2 through 8): FIG. 39A corresponds to step 2 of FIG. 38, FIG. 39Bcorresponds to step 2B of FIG. 38, FIG. 39C corresponds to step 3 ofFIG. 38, FIG. 39D corresponds to step 3B of FIG. 38, FIG. 39Ecorresponds to step 4 of FIG. 38, FIG. 39F corresponds to step 5 of FIG.38, FIG. 39G corresponds to step 5B of FIG. 38, FIG. 39H corresponds tostep 6 of FIG. 38, and FIG. 39I corresponds to step 8 of FIG. 38.

The laser ablation of amorphous silicon is performed in anon-overlapping pattern (i.e., the pulsed ablation beam spots areisolated and do not overlap). FIG. 40 is a Minority Carrier Lifetime(MCL) map of a silicon substrate after oxide ablation, specificallyshowing MCL improvement obtained when amorphous silicon is used as ahard mask. FIG. 40 shows the minority carrier lifetime map of a waferwhere the top half did not use the ablation mask scheme, while the lowerhalf used the α-Si mask scheme. No lifetime degradation was seen in thelower half of the wafer.

In order to open up the desired amount of area for selective emitter(i.e., lightly doped emitter junctions in conjunction with heavily dopedemitter contact) and selective base (i.e., lightly doped base region inconjunction with heavily doped base contact) regions, the passivationlayer may be ablated by using overlapped pulsed laser ablation spots.FIG. 41A is a optical microscope image showing four overlapped ablationrows with each ablation spot overlapping the next along the row. FIG.41B is an expanded view of the image of FIG. 41A. As may be observed inFIGS. 41A and 41B, the overlapping laser spots cause noticeable damageto the silicon substrate—thus resulting in minority carrier lifetimedrop. Laser-induced damage created by the beam overlap may be reduced bypatterning isolated/non-overlapped ablation spots. In the case ofoverlapping ablation spots the α-Si mask may have reduced siliconsubstrate protection effect, since once an area of α-Si is removed thenext overlapped laser beam will go through to the silicon substrate.Thus, in some instances utilizing an α-Si mask scheme, the ablationspots should preferably not be overlapped. FIG. 41C is an opticalmicroscope image showing isolated/non-overlapped ablation spots formedusing the same laser fluence as the ablation spots in FIG. 41A. FIG. 41Dis an expanded view of the image of FIG. 41C. As may be observed inFIGS. 41C and 41D, isolated/non-overlapped ablation spots may reducelaser damage as compared to overlapping laser spots under the same laserfluence. However, damage to the underlying semiconductor (e.g., silicon)substrate—areas of high minority carrier recombination that reduce theresulting solar cell efficiency—may still be observed in FIGS. 41C and41D.

FIGS. 42A and 42B are schematic diagrams showing two laser patterningopening and contact schemes. FIG. 42A show a laser patterning schemehaving overlapped ablation spots. In FIG. 42A the selective emitter (SE)and selective base (SB) regions opened/exposed by the laser ablation aredoped with an emitter dopant (e.g., p-type emitter such as boron-dopedemitter for n-base) and a base dopant (e.g., n-type base such asphosphorus-doped base for n-base), respectively. The contacts to theseselective emitter and base regions may then be formed by a subsequentlaser ablation step, such as that as outlined in the process flows ofFIGS. 36 and 38.

FIG. 42B shows a “spot-in-spot” laser patterning scheme where the SE andSB openings are not overlapped (i.e., isolated openings or islands) andthe contact openings are aligned inside and isolated within the SE andBase openings (e.g., having a single base contact opening per discretebase island).

FIGS. 43A and 43B are scanning electron micrographs of a spot-in-spotlaser pattern. FIG. 42A shows the selective emitter (SE) and base (B)openings and FIG. 42B, shows emitter and base contacts centrally locatedinside the SE and base openings, respectively. As can be observed,although the spot-in-spot technique reduces laser-induced damage ascompared to overlapped spots, some crystalline lattice damage is stillpresent and may reduce solar cell efficiency due to recombinationlosses. In some instances, this efficiency loss may be negligible.However, in other cases it may be desired to further reduce and/oreliminate this damage and improve the solar cell efficiency.

The solar cell cross-sectional diagrams of FIGS. 39A through 39I show asolar cell with spot in spot laser patterning.

For the laser beam having the Gaussian energy distribution across thebeam cross section, thicker films may be required to absorb the peakenergy in the center of the Gaussian. The use of the flat top energyprofile as shown in FIG. 27B may be utilized allowing for a thinner hardmask layer to be used. While short pulse length in the picosecond rangeare suitable for absorption in thin films of amorphous silicon, the useof the flat top pulse energy profile enables the use of nanosecond pulselength beam. For the nanosecond flat top beam a shorter wavelength suchas UV is may be used.

Further, beam spot shapes shown herein are circular. In some instances,a square or rectangular beam spot shape may be utilized to minimizeoverlapping laser damage and may be particularly advantageous in theformation of continuous emitter or base lines or contact openings.

FIGS. 44A and 44B are cross-sectional diagrams of a solar cell having aninterdigitated orthogonal back contact metallization pattern with twolevels metallization (e.g., on-cell metallization metal 1 patternedorthogonally to metal 2, metal 1 and metal 2 separated by an insulatingbackplane), and spot in spot laser patterning, such as may be formedusing the process flow of FIGS. 36 and 38. FIG. 44A is a cellcross-section showing metal 1 and metal 2 emitter contact and FIG. 44Bis a cell cross-section showing metal 1 and metal 2 base contact. Thebackside passivation layer may comprise, for example, an APCVD depositedBSG/USG/PSG/USG dielectric layer stack. The cell frontside may betextured and coated with a passivation layer (e.g., PECVD hydrogenatedsilicon nitride, AlOx/hydrogenated silicon nitride, or amorphoussilicon).

And while the use of a single α-Si layer for both selective emitter andbase opening has been described, other schemes may use several α-Silayers, for example, one before selective emitter ablation and anotherbefore the base ablation. Additionally, another α-Si layer may be usedbefore the contact ablations. Alternatively, the α-Si layer is used onlyonce, at either SE or Base or Contact level ablations. In each case acomplementary ablation and wet etch scheme is implemented as will beclear to one familiar with laser ablation and wet etch operations.

An advantage of the disclosed mask is the formation of self-alignedcontacts. In other words, if contact laser ablation is misaligned (e.g.,laser contact ablation falls outside the SE or base openings), the α-Siblocks the laser beam from opening the underlying oxide. An example ofcontact laser ablation in accordance with the disclosed subject matteris shown in FIG. 39I. Thus the contacts are only opened in the area thathas been previously patterned by ablating α-Si followed by wet etchingof the underlying BSG1/USG. FIGS. 45A and 45B are schematic diagramsoutlining the self-aligned contact ablation of the disclosed subjectmatter. FIG. 45A shows a self-aligned contact ablation for a selectiveemitter (SE) opening and FIG. 45B shows self-aligned contact ablationfor a base opening consistent with process flow disclosed by the cellcross-sectionals of FIG. 39. FIG. 46 is a scanning electron micrographsof a contact opening self-aligned to the base opening (such as thatshown in FIG. 45B).

Those with ordinary skill in the art will recognize that the disclosedembodiments have relevance to a wide variety of areas in addition tothose specific examples described above. The foregoing description ofthe exemplary embodiments is provided to enable any person skilled inthe art to make or use the claimed subject matter. Various modificationsto these embodiments will be readily apparent to those skilled in theart, and the generic principles defined herein may be applied to otherembodiments without the use of the innovative faculty. Thus, the claimedsubject matter is not intended to be limited to the embodiments shownherein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

It is intended that all such additional systems, methods, features, andadvantages that are included within this description be within the scopeof the claims.

What is claimed is:
 1. A method for forming doped regions on a solarcell semiconductor substrate, comprising depositing a transparentpassivation layer on a surface of a solar cell semiconductor substrate;depositing a laser absorbing hard mask layer comprising amorphoussilicon on said transparent passivation layer; selectively ablating saidlaser absorbing hard mask layer with a pulsed laser to form a patternand exposing selected regions of said transparent passivation layer;etching said exposed selected regions of said transparent passivationlayer to said solar cell semiconductor substrate, said etching formingopenings for doping regions of said solar semiconductor substrate;depositing a doping layer on said surface of said solar cellsemiconductor substrate; and annealing said solar cell semiconductorsubstrate to form doped regions in said solar cell semiconductorsubstrate in said exposed selected regions of said laser absorbing hardmask layer.
 2. The method of claim 1, wherein said amorphous silicon isdeposited using plasma enhanced chemical vapor deposition PECVD.
 3. Themethod of claim 1, wherein said amorphous silicon is deposited usingatmospheric pressure chemical vapor deposition APCVD.
 4. The method ofclaim 3, wherein said atmospheric pressure chemical vapor depositionAPCVD uses a disilane source gas.
 5. The method of claim 1, wherein saidamorphous silicon is deposited using physical sputtering.
 6. The methodof claim 1, wherein said transparent passivation layer is atmosphericpressure chemical vapor deposition APCVD oxide.
 7. The method of claim1, wherein said transparent passivation layer is thermal oxide.
 8. Themethod of claim 1, wherein said transparent passivation layer is siliconoxynitride or a stack of SiO₂/SiOxNy.
 9. The method of claim 1, whereinsaid transparent passivation layer is aluminum oxide.
 10. The method ofclaim 9, wherein said aluminum oxide is deposited using atmosphericpressure chemical vapor deposition APCVD or atomic layer deposition ALD.11. The method of claim 1, wherein said transparent passivation layer issilicon nitride.
 12. A method for forming doped regions on a solar cellsemiconductor substrate, comprising depositing a transparent passivationlayer on a surface of a solar cell semiconductor substrate; depositing alaser absorbing hard mask layer on said transparent passivation layer;selectively ablating said laser absorbing hard mask layer with a pulsedlaser to form a pattern and exposing selected regions of saidtransparent passivation layer; etching said exposed selected regions ofsaid transparent passivation layer to said solar cell semiconductorsubstrate, said etching forming openings for doping regions of saidsolar semiconductor substrate; depositing a doping layer on said surfaceof said solar cell semiconductor substrate; and annealing said solarcell semiconductor substrate to form doped regions in said solar cellsemiconductor substrate in said exposed selected regions of said laserabsorbing hard mask layer, wherein said laser absorbing hard mask layeris a nonconductive ceramic comprising aluminum nitride.